Flow-related characteristics of a fluid, such as changes in viscosity with temperature of a liquid, are often measured to provide important information about the fluid. The speed at which blood coagulates when subjected to a coagulation reagent has particular utility in the medical field. Numerous methods have been proposed and used to measure coagulation. Some of the methods are suitable for use using disposable cuvettes or strips. For example, U.S. Pat. No. 4,797,369 to Mintz teaches a mechanical method to detect clot formation by looking for fibrin threads as a probe is pulled from a sample/reagent mixture. Other methods use magnetic stir bars or particles that are located in the sample/reagent mixture, under the influence of an oscillating magnetic field, looking for a reduction in movement as the clot forms and gels the sample. See, for example, U.S. Pat. No. 4,849,340 to Oberhardt. Another method, shown in U.S. Pat. No. 4,756,884 to Hillman, disclose monitoring the capillary flow of a whole blood sample by observing a non-stationary speckle pattern when coherent light is scattered from the cells in the moving sample; coagulation is detected when the speckle pattern becomes stationary, indicating the cessation of sample flow in the capillary.
Air pressure has been used to move samples in a cuvette for the purpose of measuring coagulation time. For example, U.S. Pat. No. 4,725,554 to Schildknecht uses air pressure to move the sample back and forth across an edge to create a clot, and then detects the formation of the clot at the edge by measuring a change in optical absorption. Other examples are shown in U.S. Pat. Nos. 3,890,098 to Moreno and 3,486,859 to Greiner where two cups or chambers are interconnected through a capillary, and air pressure is used to transfer the liquid reagent and sample combination back and forth until a clot blocks the capillary and the increase in air back pressure is detected.
U.S. Pat. No. 5,302,348 to Cusack discloses a coagulation measurement apparatus which uses disposable cuvette, one end of which is inserted into the measurement apparatus. The cuvette includes a cup-shaped sample reservoir and a pair of open-ended passageways. Each passageway opens to the ambient environment through the sample reservoir at their proximal ends and to the ambient environment at their spaced-apart distal ends. A sample drop of blood is placed in the fluid reservoir positioned external of the apparatus. The open distal end of one of the passageways is connected to a first pump which draws a fluid sample into the first passageway. A second pump is connected to the open distal end of the second passageway and draws an unused portion of the sample into the second passageway so that no sample is left in the sample receptacle. The test sample in the first passageway is caused to oscillate back and forth through a restricted area which is preferably treated to be a more efficient clotting surface. A pair of light sensors, one on each side of the restricted area, are used to determine when the leading or trailing edge of the test sample passes a sensor. Endpoint, that is when sufficient clotting has occurred, is determined when the time for the sample to traverse the restricted region is a predetermined percentage longer than an immediately proceeding time. The rate of oscillation can be adjusted throughout a run to avoid breaking apart a weak clot with a long clotting time. A heater is located under the cuvette when inserted into the measurement apparatus to set the operating temperature for the particular chemical reaction.